1. Field of the Invention
The invention is directed to a sheath wave barrier (also sometimes referred to as a sheath wave trap), preferably for employment in a feeder of a local antenna of a magnetic resonance apparatus having a winding composed ,of a coaxial line.
2. Description of the Prior Art
Sheath wave barriers prevent an infeed of external high-frequency fields onto the outer conductor of a signal line that is unbalanced to ground, for example, a coaxial line. The external fields can be coupled-in from a symmetrical antenna connected to the line or from other transmission antennas. Sheath wave barriers must therefore also be utilized in feeders to local antennas in a diagnostic magnetic resonance apparatus.
German OS 38 11 983, corresponding to U.S. Pat. No. 4,922,204, discloses a sheath wave barrier of the type described above. The inductance of the sheath wave barrier is fashioned as a toroid, whereby the winding is composed of a coaxial line. Even though the external dimensions of the sheath wave barrier can be made smaller by employing a highly permeable ferrite core, it would be desirable to make the outside dimensions of the sheath wave barrier even smaller. The risk of coupling external fields into the sheath wave barrier is thereby further reduced. The relatively high manufacturing outlay caused by the toroidal form of the winding is also a disadvantage.